This invention relates to a method and device for recording and reproducing information such as video signals and audio signals.
In recent years, an information recording and reproducing method has been known wherein information is recorded in a spiral track by applying onto a disk type recording medium a light beam, such as a laser beam, modulated by the video signals or audio signals and wherein while pursuing such recorded information track with a read-out sensor, the information on the track is read out in a non-contact manner.
In this case, whether the information to be recorded on the disk type recording medium is a video signal or an audio signal, it is represented by a pulse train subjected to an appropriate modulation, such as frequency modulation (FM), pulse width modulation (PWM) or pulse code modulation (PCM). The information is recorded on the information track so as to be readable with optical means in such a way that the recording medium has its surface made uneven or its reflection factor varied in correspondence with the "1" and "0" values of the encoded data.
The information is recorded in a spiral information track around the center of rotation of the recording medium. In this regard, usually the center of rotation during reproduction somewhat deviates from that during recording on account of a working error between the axis of rotation and the mounting hole of the recording medium. In particular, in the case of reading out the information from a replica which has been obtained by subjecting the recording medium to suitable processing, the deviation of the centers of rotation can reach approximately 100 [.mu.m]. Therefore, the radius of the information track on the replica deviates sinusoidally at an amplitude of .+-.100 [.mu.m] every revolution of the replica. Assuming that the width of the information track is about 1 [.mu.m] and that the recording density of the information tracks in the radial direction of the recording medium is about 2 [.mu.m], as many information tracks as 100 odd tracks move due to the deviation of the centers of rotation. It is accordingly indispensable to the accurate read-out of the information to control the position of the read-out sensor so that the information track may always come to the center of the visual field of the read-out sensor.
In general, in the read-out sensor, the amplitude of the reproduced information thereby obtained becomes a maximum when the sensor position coincides with the center of the information track, and it becomes smaller as the sensor position is more distant from the information track. Merely by detecting such reproduced information, however, it is difficult to discriminate the direction in which the read-out sensor has deviated with respect to the center of the information track.
There has accordingly been hitherto proposed and known a method wherein recording is made with only a synchronizing signal part of the information track set off, and an output corresponding to the direction of deviation of the read-out sensor from the information track is thus provided, whereby the sensor position can be controlled with the output. According to such prior-art method, however, a signal indicative of the polarity or phase of the offset (hereinbelow, termed "reference signal") needs to be contained in the information in the recording. This brings about such problems that a demodulator circuit for the reference signal is required in the reproduction apparatus and that the information density of the recording medium increases to render the precision condition of the playback apparatus more severe.